1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a multifunction peripheral, a printer or a facsimile machine and particularly to a technology for a calibration process.
2. Description of the Related Art
Conventionally in electrophotographic image forming apparatuses, image quality is ensured by executing a calibration process for forming test patches for image density adjustment on a surface of an intermediate transfer belt, detecting the densities of the test patches, adjusting density by adjusting a developing bias voltage, a conversion table for print data and the like based on the densities of the test patches.
For example, there is known a technology for detecting the densities of image patches formed on the transfer belt by an optical sensor and adjusting image density by adjusting a developing bias voltage based on the detected densities of the image patches.
In such a calibration process, it is necessary to measure the densities of test patches formed on the intermediate transfer belt and the densities at positions of the intermediate transfer belt where the test patches are to be formed, but not yet formed (background densities) and calculate differences between the former and latter measurement values to eliminate influences such as the luminance of the intermediate transfer belt in a state where no image is formed for accurate calculation of the densities of the test patches formed on the intermediate transfer belt.